The invention relates generally to computer system data transfer operations and, more particularly, to the detection of a data transfer termination action.
Graphics capable computer systems may be characterized by their ability to rapidly generate and manipulate graphical images. In general, graphical rendering performance improves with increased memory availability (i.e., quantity) and bandwidth (i.e., access speed). As three dimensional rendering hardware and software become more pervasive, the need for larger faster memories will likely accelerate. One approach to meeting the memory requirements of graphical processing computer systems is through the use of a special purpose memory interface. For example, the Accelerated Graphics Port (AGP) specification from Intel Corporation defines a component level interface to facilitate the use of computer system memory during graphical processing operations. (See the xe2x80x9cAccelerated Graphics Port Interface Specification,xe2x80x9d Revision 2.0, 1998).
One feature of the AGP interface is that it provides three data transfer rates: 1xc3x97, 2xc3x97, and 4xc3x97. In the 1xc3x97 transfer mode, a master device may transfer data transfer requests to a target device at a clocking frequency of 66 MHzxe2x80x94corresponding to the operational speed of a standard 66 MHz PCI-type bus. (See the xe2x80x9cPCI Local Bus Specification,xe2x80x9d Revision 2.2, 1999; available from the PCI Special Interest Group). In the 2xc3x97 transfer mode, master to target data requests are transferred at a clocking frequency of 133 MHz, and in the 4xc3x97 transfer mode, data requests transfer operations occur at a clocking frequency of 266 MHz. 2xc3x97 and 4xc3x97 transfer modes are realized through the use of xe2x80x9csidebandxe2x80x9d signals. For example, the sideband address (SBA) bus and associated sideband strobe signals may be used to enqueue AGP commands from an AGP master to an AGP target. In a typical implementation, an AGP master is a plug-in card and an AGP target is integrated within the processor-to-PCI bus bridge circuit.
Referring to FIG. 1, in accordance with the current AGP specification, AGP master 100 may transfer data requests to AGP target 102 via SBA bus 104 under control of master driven strobe signals 106. In addition, the AGP specification defines AGP clock signal 108 as the fundamental control clock for an AGP interface. AGP clock 108 is used to transfer data requests on SBA bus 104 in the 1xc3x97 mode. When in either the 2xc3x97 or 4xc3x97 transfer modes, SBA bus 104 operates at two different data rates in a time multiplexed fashion: 66 MHz and 133 MHz in the 2xc3x97 mode; 66 MHz and 266 MHz in the 4xc3x97 mode. The low-speed (66 MHz) rate is used by AGP master 100 to initiate a synchronization event by driving SBA bus 104 with a 0xc3x97 FE value. Two AGP clock cycles later, master 100 begins strobing data (via strobe signal 106) into AGP target 102 at one of the high-speed rates, 133 MHz or 266 MHz. Thereafter, AGP master 100 may continuously transfer data (i.e., data requests) to AGP target 102. Prior to terminating its transfer operation, AGP master 100 is required to drive SBA bus 104 with a NOP pattern for a minimum of four AGP clock cycles. (While a master is required to transmit at least four NOPs prior to terminating a transfer operation, such an action does not guarantee terminationxe2x80x94master 100 may resume transferring data even after it transmits the required NOPs). If AGP master 100 does terminate its data transfer operation, it is required to drive strobe signal 106 to a specified state (high, for example) for a minimum of eight AGP clock cycles. Only then may master 100 initiate another synchronization event.
At least two factors complicate the ability of AGP target 102 to determine when a SBA bus data transfer operation has been terminated by master 100. First, AGP master 100 may resume transferring data even after transmitting four or more NOPs. Thus, target 102 may not use the presence of the required NOPs to detect the end of a data transfer operation. Second, the state of SBA port signals (e.g., SBA bus 104 and strobe signal 106) are undefined relative to AGP clock 108 during SBA bus data transfer operations. Thus, the AGP clock may not be used by target 102 to directly sample the state of strobe signal 106 to detect the end of a data transfer operation. Without the ability to reliably detect when a SBA bus data transfer operation has stopped, target device 102 may not detect subsequent synchronization events. Missed synchronization events may, in turn, cause a general malfunction of the AGP interface.
Thus, it would be beneficial to provide a technique to detect when an AGP master device has terminated a SBA bus data transfer operation.
In one embodiment, the invention provides a circuit to detect when an accelerated graphics port master device terminates a sideband bus data transfer operation. The circuit includes a first register to cyclically generate a predetermined sequence of output signals at a rate determined by a first clock signal, a second register to cyclically generate the predetermined sequence of output signals at a rate determined by a second clock signal (each output signal of the second register having a corresponding first register output signal), and a detector to detect a mismatch between an output signal from the second register and a corresponding output signal from the first register. In other embodiments, the invention provides a bridge circuit and a computer system incorporating a circuit as described above.